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Repair spalled concrete pillars?


Teal
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I have a building with a fair bit of "concrete cancer", with the steel within the concrete corroding in places. I've spoken to a few people and seem to get endless products but no two people seem to recommend the same thing.

It is precast 180x100mm  pillars- and to these are attached further of the same that are rafters that form apex roof. Joints are pins  The pillars and rafters are structural. The remainder of the building is blockwork. I'm keen to get the pillars and rafters sorted and would appreciate if anyone could give any guidance on best way to do this.

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If the steel is corroding then it sounds like you may be too late for a cure.

Do you have pillars that are spalling but the steel is not exposed? Any sign of rust streaking? What is the area of the repair?

You could try applying a zinc primer to any exposed steel then filling with a repair mortar, but as a minimum you would need to scabble back your conc to sound and ideally fully expose any steel. Pictures would help but replacement of pillars with exposed steel and repair of the others would probably be my advice. 
 

Fosroc do a good range of products that I have used on major projects before.

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  • 3 weeks later...

Thanks all, sorry for the delayed reply. I attach a few photos.

I did have an engineer look at it, and he said the pillars are ok but need to hack away a bit, around any obvious corrosion treat by painting an unspecified product and then an epoxy repair mortar his suggestion, but I spoke to Fosroc and they said that wouldn't be required.

Hope the pictures helps some pillars virtually unaffected, have just photographed some of the worst, interestingly it is only the inside of the building where the concrete has spalled like this, the outside of the pillars is fine. Perhaps as used to house pigs?

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The outside of my house is worse than that. Between floor and garage. Town house built on hill. Normal two stories at back, three at roadside with garages. My surveyor said, hack off loose, paint with anti rust, red lead or similar, then a fibre glass and concrete repair. Did all that, it came away again just after the five year guarantee ran out. Now I just paint it fresh when it re rusts. Doesn't seem to stop folk buying them round here.

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If worried about the risk of structural collapse, you might consider adding internal frames between the existing concrete frames, provided that you can tolerate some loss of internal width.

Simplest DIY method would be timber stanchions bearing on the floor (kept upright by strapping them to the block walls), and rafters of the same material.   Triangular plywood gussets bolted to stanchions and rafters, making rigid joints at eaves and ridge, as in a "portal frame" design.

1786455684_Timberstanchionsraftersplywoodgussets.gif.ea025ee51b6a14d0e7f4a5e7866303a7.gif

Steel frames could be of smaller dimensions, and therefore intrude less into the building.   Best to avoid any thin-walled steel hollow section for this sort of job, because moisture rapped inside could lead to internal corrosion.

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Thanks @Centrepin and @McSpredder - just what I wanted to hear!! Will chip some of the failed stuff away and then get a tin of hammerite red oxide by the sound of it and paint it, and then will try to put new mortar on. 5 years isn't terrible.

I'm gong to put a steel collar as per your sketch on the ridge and bolt through - as one currently has a bar of timber across but the others seem ok. What spec/thickness of steel would you use?

It's all fairly sturdy as is, but just keen to slow deterioration

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3 hours ago, Teal said:

I'm gong to put a steel collar as per your sketch on the ridge and bolt through - as one currently has a bar of timber across but the others seem ok. What spec/thickness of steel would you use?

Not quite sure what that timber is doing, a photo showing the ridge connection with added timber, and one of the others that seem OK, would be helpful.

Roughly what size is the building?   Looking at the size of the rafters against the concrete blocks, I am guessing it is not very big, maybe something like a 20ft span double garage?  

Atcost put up lots of concrete framed agricultural buildings around 1960-1980, so it might be worth asking local farmers about their experience of repairs, and also looking at this link:   https://thefarmingforum.co.uk/index.php?threads/replacing-atcost-concrete-stanchion.166782/

Concrete Repair Association might have useful info, though I suspect they are mainly involved with larger projects.   https://www.cra.org.uk/

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1 hour ago, McSpredder said:

Not quite sure what that timber is doing, a photo showing the ridge connection with added timber, and one of the others that seem OK, would be helpful.

Roughly what size is the building?   Looking at the size of the rafters against the concrete blocks, I am guessing it is not very big, maybe something like a 20ft span double garage?  

TYVM will look at those linked, Yeah you are right about it being 20ft span! Long building as used to be piggery, I,ve an aerial photo of 1961 and it was on it then, but looks fairly new so that could tie in perfectly with what you are saying.

Photos attached, 3 of the crude timber and one of the apex that looks fine. A pair of collared doves have a nest on the top of every single rafter, if you notice twigs hanging out..

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Hinge pin at the ridge allows the concrete rafter to be tapered, because upper end is not subject to much bending stress.   Quite different from a modern steel portal frame, which will usually have a big gusset to make the ridge connection inflexible.

Be wary of bracing the ridge so much that it can no longer act as a hinge -- that could actually increase the bending stress on the concrete rafters, and make them more liable to failure.   I would be inclined to make any bracing from timber which can "give" a little, rather than from thick steel sections.

Must state here that although I was an engineer and had quite a bit to do with farm buildings, my qualifications were NOT in structural engineering.   Maybe there a qualified SE among the forum members?

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